add glossary link to docs nav

at the end? Not sure that's right, maybe better at the start?
reuse better text for how-it-works page
wording fix (dup. of 'later')
updates to language page (gettext)
reconcile boundaries and fms_and_mapit pages
reformat boundaries page to hide/reveal setup details

The "How to setup" information is superuseful but only to readers
who specifically need that one -- the *list* of boundary types
and why to use them is more useful with them hidden... so wrapped
them in the hide/reveal <dl> markup.

Note also fixed the MAPIT_AREA_TYPES error (it should be MAPIT_TYPES)
throughout this page; would have been better to have made changes
separately, gah.
typo, rewrite